1. Field of the Invention
This invention relates to optical elements, and more particularly to a glass body having a fluorescent pattern for use in optical instruments, for example, the fluorescent pattern existing inwardly of a surface of the glass body, and to a method of making such glass body.
2. Description of the Prior Art
Glass bodies having a graticule, reticule, grating, character, symbol or other pattern formed on a surface thereof have been widely used with various optical instruments. These patterns have heretofore been formed either by applying a pigment on a surface of a glass body or by engraving the surface of the glass body through one of several various techniques to provide recesses in such surface, and then filling the recesses with a pigment. The pattern thus formed is always exposed outwardly of the surface of the glass body, and accordingly tends to reduce the amount of light transmissive through the glass or to obstruct the field of view.
To eliminate these disadvantages, there has already been proposed a method whereby a pattern is formed on a surface of a glass body by using a fluorescent dye or pigment which is normally transparent, but which, when irradiated with exciting radiations, can produce a color. The pattern provided by such method overcomes the above-noted disadvantages in that it appears only when irradiated with exciting radiations, but is otherwise invisible, thus to maintain the glass body transparent. However, the fluorescent dye or pigment used for the pattern is always exposed so that the pattern formed thereby is susceptible to chemical and physical destruction or injury. If, in an effort to overcome these disadvantages, the surface of the glass body coated with the fluorescent dye or pigment is covered with a separate film of glass, not only the capacity of fluorescence but also the configurational accuracy of the pattern would be so severely reduced that such glass body would lose its usefulness for fine measuring instruments.